1. Field of the Technology
The disclosed technology relates generally to semiconductor films and more particularly to semiconductor films incorporating dopants at desired concentrations.
2. Description of the Related Art
Highly doped semiconductor regions find many uses in various semiconductor devices. For example, in advanced transistor scaling (e.g., below about 20 node), such highly doped semiconductor regions may be advantageously used to form abrupt junctions, for example, to reduce short channel effects, by providing ultra-shallow source and drain junctions, such that loss of channel control by the gate due to source and drain depletion regions can be minimized. In three-dimensional transistors, for example in tri-gate or fin field effect transistors (finFETs), such highly doped semiconductor regions may provide strain in the channel of the finFETs that enhance the mobility of carriers, e.g., electrons in NMOS finFETs.
Formation of such highly doped semiconductor regions by many known techniques can result in certain undesirable effects. For example, the source and drain junctions formed by traditional techniques such as ion implantation results in a Gaussian distribution of dopants having a relatively large straggle, which can limit the abruptness of the junctions. In addition, the channeling of the dopant can lead to increase in the depth of the source and the drain, leading to short channel effects. Ion implantation is also limited in certain applications due to its tendency to destroy crystal structure, particularly for heavy doping steps. In other techniques, dopants are diffused into a semiconductor layer or substrate. For example, doped semiconductor material can be deposited adjacent to a channel region of a transistor and the dopants diffused in the semiconductor to form the source and drain regions. Both diffusion doping and any dopant activation step (which is also applicable to implantation) at high temperatures (e.g., greater than 800° C.) can lead to undesirable dopant distributions with less defined junctions as well as loss of desirable strain. Forming doped amorphous and polycrystalline semiconductor layers can raise similar issues.
In situ doping of semiconductor material involves incorporation of desired dopants during deposition. This technique can be advantageous in providing dopants within the layer with very high activated fractions while maintaining junction depth profile and abruptness. For example, transistor structures can have well-defined junctions formed by in situ doped epitaxial semiconductor deposition. In situ doping can also be useful for other semiconductor structures, such as contact regions, gate electrodes, interconnects, etc. However, known techniques are limited by deposition kinetics as to the amount of dopant that can be incorporated in situ.
Thus, there is a need for improved techniques for forming doped semiconductor regions.